1. Field of the Invention
The present invention relates to the improvement of an automatic search tuner, and more particularly, to an automatic search tuner in which an erroneous stop of an automatic search operation can be prevented by an interference signal generated due to an intermodulation interference.
2. Description of Related Arts
On page 439 through 447 of a data book entitled "SANYO's 1986 and 1987 Semiconductor Integrated Circuit for Home Audio" published on Aug. 10, 1986, an IC (LM 7000) for a frequency synthesizer for use in an automatic search tuner is described. The above-described IC including a phase-locked loop (PLL) for changing a local oscillation frequency has a search function of automatically changing the dividing ratio of a programmable divider constituting the PLL in response to an automatic search signal. The IC is also provided with a search stop circuit of an IF count system to count pulses generated according to the frequency of an IF signal and generate a search stop signal when a counted value has reached a predetermined value.
FIG. 2 is a circuit diagram of an automatic search tuner constituted by using the IC. In FIG. 2, a signal received by an antenna 1 is amplified by a radio frequency (RF) amplifying circuit 2, and then, mixed with a signal outputted from a local oscillation circuit 4 so as to convert the signal into an intermediate frequency (IF) signal. The IF signal is amplified by an IF amplifying circuit 5 and detected by a detecting circuit 6 to generate a signal at the output terminal of the detecting circuit 6. The frequency of the signal produced by the local oscillation circuit 4 is controlled by the signal outputted from a frequency synthesizer 7 including a PLL circuit.
An automatic search operation is performed as follows: An automatic search-start signal is applied to the terminal 8 of the frequency synthesizer 7 so as to change the signal to be outputted from the frequency synthesizer 7. The signal outputted from the frequency synthesizer 7 is applied to the local oscillation circuit 4 comprising a voltage controlled oscillator (VCO). As a result, the frequency of the signal outputted from the local oscillation circuit 4 changes. An automatic search is performed according to the change of the local oscillation signal. When the frequency of a receiver approaches the frequency of the signal transmitted from a broadcasting station, an "L" level signal corresponding to the S-curve characteristic of the detecting circuit 6 is outputted from a muting circuit 9. The "L" level signal outputted from the muting circuit 9 is shaped by a waveform shaping circuit 10, and thereafter, triggers a counter control circuit 11. As a result, a control signal is generated from the counter control circuit 11. Upon receipt of the control signal, a buffer circuit 12 generates a pulse in accordance with the frequency of the IF signal. A counter 13 starts counting pulses upon receipt of the control signal outputted from the counter control circuit 11. When a numerical value counted by the counter 13 has come to a predetermined value (for example, a value corresponding to the IF frequency of 10.7 MHz), the counter 13 generates an automatic search stop signal. As a result, the automatic search operation stops and the receiver starts receiving the signal transmitted from the broadcasting station. When a value of the counter 13 has not reached the predetermined value, the automatic search operation continues by a predetermined channel step, i.e., a tuning frequency is increased or decreased according to the automatic search operation and the counter 13 counts pulses so as to decide whether or not a numerical value has attained a predetermined value.
The receiver is tuned to the frequency of the signal transmitted from a desired broadcasting station by the above-described automatic search operation. The synthesizer 7, the counter 13, and the counter control circuit 11 are formed in one or two microcomputers or large scale integrated circuits (LSIs). The IF amplifying circuit 5, the detecting circuit 6, the muting circuit 9, and the buffer circuit 12 are formed in an integrated circuit (IC) for use in a tuner. Therefore, the automatic search tuner shown in FIG. 2 can be composed of two or three integrated circuits.
The stop signal for stopping the automatic search operation generated by using the counter prevents the malfunction of the automatic search tuner, i.e., the automatic search operation is stopped by the receipt of an interference signal. Accordingly, a signal transmitted from the desired broadcasting station can be reliably received.
However, the automatic search tuner described above has a problem that when an interference signal having a frequency included in a signal-receiving band is generated by an intermodulation interference, the automatic search operation is stopped by the frequency of the interference signal. The frequency of the interference signal is converted into an IF signal when the frequency conversion is made by the mixing circuit. Therefore, the counter is incapable of distinguishing between the frequency of the interference signal and that of a signal transmitted from the desired broadcasting station. Accordingly, the automatic search operation stops.